The basic objective of this project is to analyze the capacities of primitive mammalian hematopoietic progenitor cells to undergo self-renewal and differentiation in relation to their position within the cell cycle. By employing cell separation techniques, both with centrifugation and at unit gravity, rapidly growing murine bone marrow will be sedimented to obtain fractions enriched in pluripotent and unipotent progenitor cells. Proliferating stem cells in each cell-cycle phase, including Go cells will thus be obtained and their capacity to undergo self-renewal tested by double in vivo spleen colony transplant experiments. The capacity of spleen colonies (CFU) for differentiation will be tested by the in vitro CFC-agar assay method, as well as the in vitro assay for cells committed to erythropoiesis. Both self-renewal and differentiation capacities will be assessed in relation to the position of CFU within the cell cycle. Unipotent cells responsive to erythropoietin will likewise be synchronized by sedimentation and the capacity of these cells to undergo erythroid differentiation assessed as a function of the cell cycle. These experiments are expected to provide insight into the relationship of progenitor cells to one another and into the nature of the capacity of progenitor cells for differentiation and self-renewal in relation to the cell cycle.